The deployment of Raman spectroscopy has been hindered by the lack of a low cost, rugged and stable semiconductor laser source that can provide high spectral brightness, where the spectral brightness is defined as laser power divided by its spectral linewidth. A broad stripe or broad area diode laser (e.g. with a stripe width typically in the range of 20-500 μm) can provide a high output power of >1 W, while its spectral linewidth is on the order of several nanometers or more due to the existence of a large number of lasing modes in the Fabry-Perot (F-P) laser cavity. This broad linewidth limits the broad stripe laser only to those low resolution Raman spectroscopy applications as disclosed by Clarke et al. in U.S. Pat. Nos. 5,139,334 and 5,982,484. On the other hand, the output power of a single mode diode laser (with a stripe width of a few micrometers) is typically limited to a few hundred milliwatts. This power level is inadequate for Raman spectroscopic analysis of some materials with not so strong Raman scattering. An example of the application of a single mode DBR laser for Raman spectroscopy can be found in U.S. Pat. No. 5,856,869 disclosed by Cooper et al.
Recently, it has been demonstrated that an external cavity laser (ECL) structure can be used to narrow down the linewidth of a broad stripe laser as disclosed by Smith et al. in U.S. Pat. No. 6,100,975 and by Tedesco et al. in U.S. Pat. No. 6,563,854. In these references, the lasing wavelength of the diode laser is locked by an external grating that is positioned a distance away from the semiconductor laser chip to form an external cavity. However, the ECL structure is relatively complicated and the long cavity length may result in mechanical and thermal instability.
Therefore, there is a need for an improved Raman spectroscopic apparatus that utilizes a more rugged and stable semiconductor laser source with high spectral brightness to provide better sensitivity and spectral resolution.